Experiments are proposed that should lead to an understanding of DNA:protein interactions during bacteriophage lambda DNA packaging. Major topics include: (1) study of the recognition steps by which lambda DNA is selected for packaging from a DNA pool that also includes bacterial DNA, and (2) study of the interactions involved in cohesive and site cleavage. The role of base sequences in cos cleavage and recognition will be examined by study of (1) a cohesive end site mutant of lambda and (2) a variety of revertants of the mutant, some of which appear to have acquired cos function by acquisition of host sequences, and (3) the packaging incompatibility between lambda and phage 21. The role of particular phage proteins will be studied by isolation of mutants with altered packaging specificity. Additional studies involve isolation of more cos mutants, a test of the functional significance of the rotationally-symmetrical base pairs of cos, and in vitro assembly studies on recognition steps.